1. Technical Field
The present invention relates broadly to the replenishment of metals in aqueous electrolyte solutions. The present invention will be particularly described with respect to the replenishment of tin in an acidic electrotinning bath, wherein the electrotinning is carried out with an insoluble anode depleting tin ions from the bath.
2. Description of the Prior Art
U.S. Pat. No. 4,181,580 describes a process for electrotinning steel strip in an electrolytic bath. The steel strip is the cathode and the anode is an insoluble metal plate positioned in the bath. The patent discloses several advantages achieved by the use of an insoluble anode rather than a soluble anode. However, an insoluble anode requires that the tin in the electrolytic bath be replenished. In U.S. Pat. No. 4,181,580, this is accomplished by withdrawing electrolyte from the electrolytic bath to a reactor which is exterior to the bath. The reactor contains a bed of tin in particulate form. Oxygen is introduced into the reactor and reacts with the tin to dissolve the tin. The rate of dissolution of the tin is controlled by the amount of oxygen which is introduced into the reactor. The rate of dissolution maintains the concentration of dissolved tin in the electrolytic bath at a desired level.
A primary problem with this process is that the oxygen also promotes the reaction of dissolved Sn.sup.+2 to Sn.sup.+4 so that an amount of dissolved tin is converted into a sludge which has to be removed from the electrolyte. This requires the use of a separate sludge removal system.
U.S. Pat. No. 4,789,439 discloses a process which purports to avoid the need for a sludge removal system. In this process, electrolyte is withdrawn from an electrolytic tinning bath and is fed into the anode chamber of an electrolytic cell. The anode chamber contains a bed of tin particles. The cathode and anode chambers are separated by a tin impermeable membrane. A power source connected to the electrolytic cell provides an electric current by which tin ions are formed electrolytically in the reaction EQU Sn.fwdarw.Sn.sup.+2 +2e.sup.-
and are added to the electrolyte.
One problem with this process is that an external power source is needed, to drive the reaction, and this adds to the cost of electrotinning. In addition, efficient operation of the electrolytic cell requires that the tin particles be in good contact with each other for the flow of current. If the particles are not in good contact, the cell resistance is increased. This causes the potential at the anode to increase, which can result in the evolution of oxygen at the anode and formation of Sn.sup.+4 and tin sludge.
U.S. Pat. No. 3,793,165 discloses an electrochemical cell for electrowinning a metal from an acidic salt solution of the metal. A cathode is immersed in the salt solution, and the salt solution functions as the cell catholyte. A gas diffusion electrode functions as the cell anode. The anolyte is an acid such as sulfuric acid. Hydrogen is introduced on the gas side of the anode. A diffusion diaphragm permeable to the anolyte separates the anode from the cell catholyte. When the anode and cathode are electrically connected together, the metal is reduced at the cell cathode depositing on the cathode. The electrodeposition occurs without the need for an external power source. The process is suitable for electrowinning metals below hydrogen in oxidation potential, such as copper or zinc. Similar subject matters are disclosed in related U.S. Pat. Nos. 4,293,396 and 4,614,575.